Which of these was NOT a part of Der Ring des Nibelungen?

Der Ring des Nibelungen, composed by Richard Wagner, is a monumental four-opera cycle that includes the works Das Rheingold, Die Walküre, Siegfried, and Götterdämmerung. The first opera, Das Rheingold, sets the stage for the entire cycle, introducing key characters and themes. Die Walküre continues the narrative, exploring the relationships between gods and mortals, while Siegfried follows the hero's adventures and challenges.

Rienzi, on the other hand, is not part of this cycle. It is an earlier opera by Wagner, inspired by the life of Cola di Rienzi, a historical figure who attempted to revive the glory of the Roman Republic. While Rienzi shares Wagner's characteristic style and thematic elements, it exists independently from the narrative structure and characters of Der Ring des Nibelungen. Thus, it is correct to identify it as the option that does not belong to the Ring cycle.